Conventional equipments projected to obtain fundus images are mainly based on a technique widely known and used in ophthalmology, which predicts the wide and uniform lighting of the fundus wall, followed by frontal capture of the reflected light in this process. The light beam used for lighting should according to this technique, penetrate in the back portion of the eyeball through its peripheryc region, allowing that the region near to the optical axis be free of intense lighting, in order to not contaminate with spurious lighting the reflected beam that returns bringing the image of the fundus.
In order to achieve these objectives, it was adopted for the lighting beam, as more appropriate an annular form, since it has a cylindrical symmetry, so, allowing distribute the light beam on the whole circle of the pupil periphery. This annular beam must have its focal position adjusted in the iris region, which coincides with the pupil plan, and shows in this plan a diameter slightly less than the pupillary opening. However, said beam must also have a elevated divergence from its focal position, to reach the fundus with a wide and uniform energy distribution.
The lighting beam should have necessarily an elevated intensity, so that a significant quantity of scattered light by the fundus returns through the pupillary opening, and be sufficient to sensitive the used camera. This is necessary because the fundus wall has the characteristics of a diffuse irradiator, working better as a scatter surface as a light-reflecting surface. The light reflectivity is a function of the surface properties, the incidence angle, and light wavelength, being more accented in higher wavelengths, near to the red visible limit. So, the fundus wall has a low inherent reflectivity.
This necessary elevated intensity is another good reason that the lighting beam presents a ring form, because the excessive spatial concentration of the lighting beam energy would affect in a adverse form the intraocular medium, which it is formed by the cornea, anterior chamber, iris, lens and pupillary opening. The annular form provides a regular distribution of light energy on the whole extension of the peripheral line of the frontal part of the eye, even that it is considered that the line of light ring has a narrow width in its focal position. This method also allows a more uniform lighting of fundus by the cylindrical symmetry that the ring form provides. The geometry of the construction and the depth of focus should be adjusted in order to obtain the most wide and uniform possible lighting.
The simultaneous capturing of the scattered light by the fundus portions undergone to this lighting is carried out, still according with this technique, with an optical system and a device to register the image. In some old equipment models the lighting beam has not the ring form, but a simple beam form that reaches the eye in the pupil periphery and projects the light straightly in a small region of the fundus. This beam has a small angle in relation to the eye optical axis, whereas the capturing is generally frontal. In these equipment models the inspection of fundus is carried out by regions, which increases the examination difficulty and increases the risk for the patient due to excessive intensity of a punctual beam.
The anterior part of the eye and the intraocular medium have differentiated structures, in whose interfaces occurs an accented scattering of incident light, this is the principal reason for which the lighting must pass this region by its periphery and in a beam form as narrow as possible, to avoid possible scattering in these structures may contribute with undesirable light stains in the image fundus. The appearance of reflexes from the peripheral region is reduced because the annular beam reaches the cornea in a remote point of the optical axis, in which the incidence angle away from the normal to the surface makes that the principal reflection be launched far from of the optical axis, and out of the capturing lens.
The means must have a part of its optical paths in common, due to an inevitable spatial superposition of both beams in the intraocular medium and in the region immediately in front of the eye, causing that the capturing lens should capture the light coming from the fundus and focalize the lighting beam in the pupil plane. The most used solution in order to combine said optical means, is to put between the capturing lens and the others means components, an oblique mirror having a central hole, which function is allow the partial sharing of the optical axis by both optical means, so that the capturing beam passes by the central hole of the mirror and propagates backwards, while the lighting beam fall on obliquely on the mirror being reflected from that to the eye.
The mirror position is calculated in order to be in the focus of a primary and real image of the light ring projected by a first optical set of the lighting means, and also be conjugated to the iris position by the capturing lens. The image formed in the mirror by the first optical set is generally produced by placing two screens shutters in the same plan, one circular and other with a circular hole, whose diameter is a bit higher than the first one. The first optical set of the lighting means must be used to focus the ring in a position near to the mirror plan.
As the lighting should be intense and sufficient to that the reflected light by the fundus be above of the threshold sensibility of the image sensor, and as the intensity of the return beam is less than the intensity of the lighting beam, it is essential remove the reflections of the most intense beam. The principal reflections take place in the cornea, in the eye internal interfaces and in the lenses used in common by the two optical means, because the reflected light may contaminate the capturing beam, causing stains and brighten points in the image, besides elevating the bottom intensity level of the image.
The basic proceeding adopted to remove the reflections in this model, is the use of polarizers placed in the lighting and capturing means, in that the polarizer of the capturing means is orientated perpendicularly to that placed in the lighting means, that is, the fundus is illuminated by polarized light and the capturing means receives light only with crossed polarization. The polarizer of the lighting means is constituted of an object with an annular form, with dimensions slight bigger than the thickness of the ring light, and placed in front of the outlet end of the optical fibers arrangement.
The technique is based on a physical principle that establishes the properties of light interaction with the material means, and defines the effects on the light in accordance with the characteristics of the material. The materials which surface is polished, or smooth, reflect light with higher efficiency, because the most part of the bright energy is reflected by an angle equal to the incidence angle, in a so called situation of speculate reflection, while materials with rough, or wrinkled surface, reflect light in a diffuse way, scattering the incident radiation in a wide angular band. These phenomena have consequences on the polarization of incident light, so that the speculate surfaces reflect light with polarization almost equal to the incident light, at least for angles near to normal, while the diffuse surfaces not polarizes the incident radiation, reflecting light with random polarization.
This physical process establishes a criterion to distinguish the light originating from the fundus from that originated by reflections coming from speculate surfaces of the eye and from the optical means, since the crossed polarizer of the capturing means blocks all light with parallel polarization to the lighting beam, allowing that only the light with perpendicular, or orthogonal polarization, to the lighting beam, passes. The capturing beam has a considerable fraction of light that passes by the polarizer, since its polarization has a random distribution, and so, always has a parallel component to the polarizer direction. To reach efficiency the system must be optimized so that the light beams pass the most lenses surfaces in angles near to a normal of the surface, so that the polarization phenomenon by reflection does not affect the system, since this phenomenon is characteristic of angles near to Brewster.
There are varied light source used in the construction of lightning means, such as incandescent lamps and halogens, which have a wide spectral band and relatively high intensity in wavelengths in the visible band, from infrared and of ultraviolet. Tungsten incandescent lamps are more stable and have more simple operation, while the halogen lamps support better a continuous operation regime. The spectral curve of halogen lamps is shifted for shorter wavelengths in relation to incandescent lamps, presenting a reasonable intensity in the ultraviolet band, although this curve vary during the use, its intensity remains quite stable. The fluorescent light, as the high-pressure xenon, also having a wide spectral band are used in more complex and expensive equipments. All these lamps present common difficulties, as the excessive divergence of irradiated light that makes necessarily the use of diaphragms, the low energetic efficiency accompanied by high heat dissipation, and the compulsory placing of filters to cut undesirable wavelengths in determined examinations. The divergence makes difficult the beam collimation and the efficiency in the capturing of emitted energy, producing not uniform lighting in the fundus if the optical means is not well corrected. The necessity of illuminating a wide region in the fundus demands a light source with a relatively big emission surface, positioned in the focal point of the lighting means, due to placing of screens shutters, also big, with the ring form.
The light emitting diodes (LED) and laser diodes also can be used alternatively as light source, since they present less divergence in the light emission. But the predominant necessity of wide spectral band and elevated intensity makes difficult the option for these components. The use of light emitting diodes (LED) is extremely advantageous as a low cost option for multiple images acquisition, using the light pulses (flashes) synchronized to the acquisitions, because the others light sources demands an excessive wait time for the electronic reload.